Yes, it varies a little from person to person. The more HCG, the darker the line, but generally the longer you're pregnant, the darker the line (as your levels go up.)

I would say it's this. The range for say 14dpo is 5-like 500 or something on blood tests, so the same person can be the same DPO but one have a level of 5 which would mean a very faint line vs a level of 100+ which would be a blaring line